Cathode materials for secondary (rechargeable) lithium batteries

ABSTRACT

The invention relates to materials for use as electrodes in an alkali-ion secondary (rechargeable) battery, particularly a lithium-ion battery. The invention provides transition-metal compounds having the ordered-olivine, a modified olivine, or the rhombohedral NASICON structure and the polyanion (PO 4 ) 3−  as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and are included to demonstrate further certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.

FIG. 1. FIG. 1 shows a typical polarization curve for the battery voltage V vs. the I delivered across a load. The voltage drop (V_(oc)-V)=η(I) of a typical curve is a measure of the battery resistance Rb(I). The interfacial voltage drops saturate in region (i). The slope of the curve in region (ii) is dV/dI≈R_(el)+R_(c)(A)+R_(c)(C), the sums of the electrolyte resistance R_(el) and the current-collector resistances at the anode and cathode. Region (iii) is diffusion-limited. At the higher currents I, normal processes do not bring ions to or remove them from the electrode/electrolyte interfaces rapidly enough to sustain an equilibrium reaction.

FIGS. 2A, 2B and 2C. FIG. 2A shows discharge/charge curves at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(1−x)FePO₄ as cathode and lithium as anode. A plateau at 3.4V corresponds to the Fe³⁺/Fe²⁺ redox couple relative to the lithium anode. FIG. 2B shows discharge/charge curves at 0.05 mA·cm⁻² (1.13 mA·g⁻¹) for the olivine Li_(1−x)Fe_(0.5)Mn_(0.5)PO₄ as cathode relative to a lithium anode. A plateau at 3.4V corresponds to the Fe³⁺/Fe²⁺ redox couple relative to the lithium anode. A plateau at 4.1 V corresponds to the Mn³⁺/Mn²⁺ couple. FIG. 2C shows discharge/charge curves vs. lithium at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(1−x)FePO₄.

FIG. 3. FIG. 3 shows discharge/charge curves of an FePO₄/LiClO₄+PC+DME/Li coin cell at 185 mA·g⁻¹ for FePO₄ prepared by chemical extraction of Li (delithiation) from LiFePO₄.

FIG. 4. FIG. 4 shows a schematic representation of the motion of LiFePO₄/FePO₄ interface on lithium insertion in to a particle of FePO₄.

FIGS. 5A and 5B. FIG. 5A shows the rhombohedral R3c (NASICON) framework structure of Li Fe₂(PO₄)₃ prepared by ion exchange from Na₃Fe₂(PO₄)₃; FIG. 5B shows the monoclinic P2₁/n framework structure of Li₃Fe₂(PO₄)₃ prepared by solid-state reaction. The large, open three-dimensional framework of FeO₆ octahedra and PO₄ tetrahedra allows an easy diffusion of the lithium ions.

FIGS. 6A and 6B. FIG. 6A shows discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for rhombohedral Li_(3+x)Fe₂(PO₄)₃ where 0<x<2. The shape of the curve for lithium insertion into rhombohedral Li_(3+x)Fe₂ (PO₄)₃ is surprisingly different from that for the monoclinic form. However, the average V_(oc) at 2.8 V remains the same. The Li⁺-ion distribution in the interstitial space appears to vary continuously with x with a high degree of disorder. FIG. 6B shows discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for monoclinic Li_(3+x)Fe₂(PO₄)₃ where 0<x<2.

FIGS. 7A and 7B. FIG. 7A shows discharge curves vs. a lithium anode at current densities of 0.05-0.5 mA·cm⁻² for rhombohedral Li_(3+x)Fe₂(PO₄)₃. A reversible capacity loss on increasing the current density from 0.05 to 0.5 mA·cm⁻² is shown. This loss is much reduced compared to what is encountered with the monoclinic system. FIG. 7B shows discharge curves at current densities of 0.05-0.5 mA·cm⁻² for monoclinic Li_(3+x)Fe₂(PO₄)₃.

FIG. 8. FIG. 8 shows discharge/charge curves at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the rhombohedral Li₂NaV₂(PO₄)₃. Rhombohedral Li₂NaV₂(PO₄)₃ reversibly intercalates 1.5 Li per formula unit for a discharge capacity of 100 mAh·g⁻¹ with average closed-circuit voltage of 3.8 V vs. a lithium anode.

FIG. 9. FIG. 9 illustrates the solid-state synthesis of Li₂NaV₂(PO₄)₃ having the rhombohedral NASICON framework.

FIG. 10. FIG. 10 shows discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for rhombohedral Li_(1+x)Fe₂(PO₄) (SO₄)₂ where 0≦x≦2.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Present-day secondary (rechargeable) lithium batteries use a solid reductant as the anode, or negative electrode, and a solid oxidant as the cathode, or positive electrode. It is important that the chemical reactions at the anode and cathode of a lithium secondary battery be reversible. On discharge, the metallic anode supplies Li⁺ ions to the Li⁺-ion electrolyte and electrons to the external circuit. The cathode is a host compound into/from which the working Li⁺ ion of the electrolyte can be inserted/extracted reversibly as a guest species over a large solid-solubility range (Goodenough 1994). When the Li⁺ ions are inserted as a guest species into the cathode, they are charge-compensated by electrons from the external circuit. On charge, the removal of electrons from the cathode by an external field releases Li⁺ ions back to the electrolyte to restore the parent host structure. The resultant addition of electrons to the anode by the external field attracts charge-compensating Li⁺ ions back into the anode to restore it to its original composition.

The present invention provides new materials for use as cathodes in lithium secondary (rechargeable) batteries. It will be understood that the anode for use with the cathode material of the invention may be any lithium anode material, such as a reductant host for lithium or elemental lithium itself. Preferably, the anode material will be a reductant host for lithium. Where both the anode and cathode are hosts for the reversible insertion or removal of the working ion into/from the electrolyte, the electrochemical cell is commonly called a “rocking-chair” cell. An implicit additional requirement of a secondary battery is maintenance not only of the electrode/electrolyte interfaces, but also of electrical contact between host particles, throughout repeated discharge/recharge cycles.

Since the volumes of the electrode particles change as a result of the transfer of atoms from one to another electrode in a reaction, this requirement normally excludes the use of a crystalline or glassy electrolyte with a solid electrode. A non-aqueous liquid or polymer electrolyte having a large energy-gap window between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) is used with secondary lithium batteries in order to realize higher voltages. For example, practical quantities of very ionic lithium salts such as LiClO₄, LiBF₄ and LiPF₆ can be dissolved in empirically optimized mixtures of propylene carbonate (PC), ethylene carbonate (EC), or dimethyl carbonate (DMC) to provide acceptable electrolytes for use with the cathodes of the invention. It will be recognized by those of skill in the art that the (ClO₄)⁻ anion is explosive and not typically suitable for commercial applications.

General Design Considerations

The power output P of a battery is the product of the electric current I delivered by the battery and the voltage V across the negative and positive posts (equation 1).

P=IV   (1)

The voltage V is reduced from its open-circuit value V_(oc) (I=0) by the voltage drop IR_(b) due to the internal resistance R_(b) of the battery (equation 2).

V=V _(oc) −IR _(b)   (2)

The open-circuit value of the voltage is governed by equation 3.

V _(oc)=(μ_(A)−μ_(C))/(−nF)<5V   (3)

In equation 3, n is the number of electronic charges carried by the working ion and F is Faraday's constant. The magnitude of the open-circuit voltage is constrained to V_(oc)<5V not only by the attainable difference μ_(A)−μ_(C) of the electrochemical potentials of the anode reductant and the cathode oxidant, but also by the energy gap E_(g) between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of a liquid electrolyte or by the energy gap E_(g) between the top of the valence band and the bottom of the conduction band of a solid electrolyte.

The chemical potential μ_(A), which is the Fermi energy ε_(F) of a metalic-reductant anode or the HOMO of a gaseous or liquid reductant, must lie below the LUMO of a liquid electrolyte or the conduction band of a solid electrolyte to achieve thermodynamic stability against reduction of the electrolyte by the reductant. Similarly, the chemical potential μ_(A)−μ_(C), which is the LUMO of a gaseous or liquid oxidant or the Fermi energy of a metallic-oxidant cathode, must lie above the HOMO of a liquid electrolyte or the valence band of a solid electrolyte to achieve thermodynamic stability against oxidation of the electrolyte by the oxidant. Thermodynamic stability thus introduces the constraint

μ_(A)−μ_(C) ≦E _(g)   (4)

as well as the need to match the “window” E_(g) of the electrolyte to the energies μ_(A) and μ_(C) of the reactants to maximize V_(oc). It follows from equations 1 and 2 that realization of a high maximum power P_(max) (equation 5) requires, in addition to as high a V_(oc) as possible, a low internal battery resistance Rb (see equation 6).

P_(max)=I_(max) V_(max)   (5)

R _(b) =R _(el) +R _(in)(A)+R _(in)(C)+R _(c)(A)+R _(c)(C)   (6)

The electrolyte resistance R_(el) to the ionic current is proportional to the ratio of the effective thickness L to the geometrical area A of the interelectrode space that is filled with an electrolyte of ionic conductivity σ_(i) (equation 7).

R _(el)=(L/σ _(i) A)   (7)

Since ions move diffusively, σ_(i) (see equation 8) increases with temperature. A σ_(i)≦0.1 Scm⁻¹ (the maximum σ_(i) represents the room-temperature protonic conductivity σ_(H) in a strong acid) at an operating temperature T_(op) dictates the use of a membrane separator of large geometrical area A and small thickness L.

σLi=(B/T) exp (−E _(v) /kT)   (8)

The resistance to transport of the working ion across the electrolyte-electrode interfaces is proportional to the ratio of the geometrical and interfacial areas at each electrode:

R _(in) ˜A/A _(in)   (9)

where the chemical reaction of the cell involves ionic transport across an interface, equation 9 dictates construction of a porous, small-particle electrode. Achievement and retention of a high electrode capacity, i.e., utilization of a high fraction of the electrode material in the reversible reaction, requires the achievement and retention of good electronic contact between particles as well as a large particle-electrolyte interface area over many discharge/charge cycles. If the reversible reaction involves a first-order phase change, the particles may fracture or lose contact with one another on cycling to break a continuous electronic pathway to the current collector.

Loss of interparticle electrical contact results in an irreversible loss of capacity. There may also be a reversible capacity fade. Where there is a two-phase process (or even a sharp guest-species gradient at a diffusion front) without fracture of the particles, the area of the interface (or diffusion front) decreases as the second phase penetrates the electrode particle. At a critical interface area, diffusion across the interface may not be fast enough to sustain the current I, so not all of the particle is accessible. The volume of inaccessible electrode increases with I, which leads to a diffusion-limited reversible capacity fade that increases with I. This problem becomes more important at lower ionic conductivity σ_(Li).

The battery voltage V vs. the current I delivered across a load is called the polarization curve. The voltage drop (V_(oc)−V)≡η(I) of a typical curve, FIG. 1, is a measure of the battery resistance (see equation 10).

R _(b)(I)=η(I)/I   (10)

On charging, η(I)=(V_(app)−V_(oc)) is referred to as an overvoltage. The interfacial voltage drops saturate in region (i) of FIG. 1; therefore in region (ii) the slope of the curve is

dV/dI=R _(el) +R _(c)(A)+R _(c)(C)   (11)

Region (iii) is diffusion-limited; at the higher currents I, normal processes do not bring ions to or remove them from the electrode/electrolyte interfaces rapidly enough to sustain an equilibrium reaction.

The battery voltage V vs. the state of charge, or the time during which a constant current I has been delivered, is called a discharge curve.

Cathode Materials

The cathode, or positive electrode, material of the present invention, for use in a secondary lithium battery, consists of a host structure into which lithium can be inserted reversibly. The maximum power output, P_(max) (see equation 5) that can be achieved by a cell depends on the open-circuit voltage V_(oc)=ΔE/e and the overvoltage η(I) at the current I_(max) of maximum power

V _(max) =V _(oc)−η(I)   (12)

ΔE is the energy difference between the work function of the anode (or the HOMO of the reductant) and that of the cathode (or the LUMO of the oxidant). In order to obtain a high V_(oc), it is necessary to use a cathode that is an oxide or a halide. It is preferable that the cathode be an oxide in order to achieve a large V_(oc) and good electronic conductivity. To minimize η(I_(max)), the electrodes must be good electronic as well as ionic conductors and they must offer a low resistance to mass transfer across the electrode/electrolyte interface. To obtain a high I_(max), it is necessary to have a large electrode/electrolyte surface area. In addition, where there is a two-phase interface within the electrode particle, the rate of mass transfer across this interface must remain large enough to sustain the current. This constraint tends to limit the electrode capacity more as the current increases.

Oxide host structures with close-packed oxygen arrays may be layered, as in Li_(1−x)CoO₂ (Mizushima, et al. 1980), or strongly bonded in three dimensions (3D) as in the manganese spinels Li_(1−x)[Mn₂]O₄ (Thackeray 1995; Thackeray et al. 1983; Thackeray et al. 1984; Guyomard and Tarascon 1992; and Masquelier et al. 1996). Li intercalation into a van der Waals gap between strongly bonded layers may be fast, but it can also be accompanied by unwanted species from a liquid electrolyte. On the other hand, strong 3D bonding within a close-packed oxygen array, as occurs in the spinel framework [Mn₂]O₄, offers too small a free volume for the guest Li⁺ ions to have a high mobility at room temperature, which limits I_(max). Although this constraint in volume of the interstitial space makes the spinel structure selective for insertion of Li⁺ ions, it reduces the Li⁺-ion mobility and hence Li⁺-ion conductivity σ_(Li). The oxospinels have a sufficiently high σ_(Li), to be used commercially in low-power cells (Thackeray et al., 1983) but would not be acceptable for the high power cells of the insertion.

The present invention overcomes these drawbacks by providing cathode materials containing larger tetrahedral polyanions which form 3D framework host structures with octahedral-site transition-metal oxidant cations. In the cathode materials of the invention having the NASICON structure, the transition-metal ions are separated by the polyanions, so the electronic conductivity is polaronic rather than metallic. Nevertheless, the gain in σ_(Li) more than offsets the loss in electronic conductivity.

Variation of the energy of a given cation redox couple from one compound to another depends on two factors: (a) the magnitude of the crystalline electric field at the cation, which may be calculated for a purely ionic model by a Madelung summation of the Coulomb fields from the other ions present, and (b) the covalent contribution to the bonding, which may be modulated by the strength of the covalent bonding at a nearest-neighbor counter cation. The stronger is the negative Madelung potential at a cation, the higher is a given redox energy of a cation. Similarly the stronger is the covalent bonding of the electrons at a transition-metal cation, the higher is a given redox energy of that cation. The lower the redox energy of the cation host transition-metal ion, the larger is V_(oc).

The redox couples of interest for a cathode are associated with antibonding states of d-orbital parentage at transition-metal cations M or 4f-orbital parentage at rare-earth cations Ln in an oxide. The stronger is the cation-anion covalent mixing, the higher is the energy of a given LUMO/HOMO redox couple. Modulation of the strength of the cation-anion covalence at a given M or Li cation by nearest-neighbor cations that compete for the same anion valence electrons is known as the inductive effect. Changes of structure alter primarily the Madelung energy as is illustrated by raising of the redox energy within a spinel [M₂]O₄ framework by about 1 eV on transfer of Li⁺ ions from tetrahedral to octahedral interstitial sites. Changing the counter cation, but not the structure, alters primarily the inductive effect, as is illustrated by a lowering of the Fe³⁺/Fe²⁺ redox energy by 0.6 eV on changing (MoO₄)²⁻ or (WO₄)²⁻ to (SO₄)²⁻ polyanions in isostructural Fe₂(XO₄)₃ compounds. Raising the energy of a given redox couple in a cathode lowers the voltage obtained from cells utilizing a common anode. Conversely, raising the redox energy of an anode raises the cell voltage with respect to a common cathode.

The invention provides new cathode materials containing oxide polyanions, including the oxide polyanion (PO₄)³⁻ as at least one constituent, for use in secondary (rechargeable) batteries. For example, the cathode materials of the present invention may have the general formula LiM(PO₄) with the ordered olivine structure, the modified olivine structure of LiM(PO₄), or the more open rhombohedral NASICON framework structure. The cathode materials of the present invention have the general formula LiM(PO₄) for the ordered olivine structure, the general formula Li_(x+y)M_(1-(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r) [PO₄]_(1-(p+s+v)) [SO₄]_(p) [SiO₄]_(s) [VO₄]_(v) for the modified olivine structure or Y_(x)M₂(PO₄)_(y)(XO₄)_(3-y) for the rhombohedral NASICON framework structure. The parameters for the modified olivine structure are as follows:

-   -   M may be Fe²⁺ or Mn²⁺ or mixtures thereof;     -   D may be a metal in the +2 oxidation state, preferably Mg²⁺,         Ni²⁺, Co²⁺, Zn²⁺, Cu²⁺, or Ti²⁺;     -   T may be a metal in the +3 oxidation state, preferably Al³⁺,         Ti³⁺, Cr³⁺, Fe³⁺, Mn³⁺, Mn³⁺, Ga³⁺, Zn³⁺, or V³⁺;     -   Q may be a metal in the +4 oxidation state, preferably Ti⁴⁺,         Ge⁴⁺, Sn⁴⁺, or V⁴⁺;     -   R may be a metal in the +5 oxidation state, preferably V⁵⁺,         Nb⁵⁺, or Ta⁵⁺.

In this preferred embodiment x is the degree of intercalation during operation of the electrode material; y is the fraction of lithium ions on the initial Fe²⁺ sites; d is the fraction of divalent ions (noted as D) on the initial Fe²⁺ sites; t is the fraction of trivalent ions (noted as T) on the initial Fe²⁺ sites; q is the fraction of tetravalent ions (noted as Q) on the initial Fe²⁺ sites; r is the fraction of pentavalent ions (noted as R) on the initial Fe²⁺ sites; p is the fraction of hexavalent sulfur (as discrete SO₄ ²⁻ tetrahedra) on the initial P⁵⁺ sites; and s is the fraction of tetravalent silicon (as discrete SiO₄ ²⁻ tetrahedra) on the initial P⁵⁺ sites; v is the stoichiometric coefficient for V⁵⁺ residing in tetrahedral sites; and M, D, T, Q and R reside in octahedral sites.

The conditions for site occupancy and electroneutrality imply:

0≦x≦1;

y+d+t+q+r≦1;

p+s+v≦1; and

3+s−p=x−y+2+t+2q+3r.

Generally, x, y, d, t, q, r, p, s, and v may be between 0 (zero) and 1 (one), with at least one of y, d, t, q, r, p, s, or v differing from 0. Preferably, y, d, t, q, r and v may vary between 0 (zero) and 0.2 ( 2/10) and p and s may vary between 0 (zero) and 0.5 (½).

In the rhombohedral NASICON framework structure, 0<y≦3, M is a transition-metal atom, Y is Li or Na and X═Si, As or S and acts as a counter cation.

The olivine structure of Mg₂SiO₄ consists of a slightly distorted array of oxygen atoms with Mg²⁺ ions occupying half the octahedral sites in two different ways. In alternate basal planes, they form zigzag chains of corner-shared octahedra running along the c-axis and in the other basal planes they form linear chains of edge-shared octahedra running also along the c-axis.

In the ordered LiMPO₄ olivine structures of the invention, the M atoms occupy the zigzag chains of octahedra and the Li atoms occupy the linear chains of the alternate planes of octahedral sites. In this embodiment of the present invention, M is preferably Mn, Fe, Co, Ni or combinations thereof. Removal of all of the lithium atoms leaves the layered FePO₄-type structure, which has the same Pbnm orthorhombic space group. These phases may be prepared from either end, e.g., LiFePO₄ (triphylite) or FePO₄ (heterosite), by reversible extraction or insertion of lithium.

FIG. 2A, FIG. 2B and FIG. 2C show discharge/charge curves vs. lithium at 0.05 mA·cm⁻² (0.95 mA·g⁻¹ and 1.13 mA·g⁻¹, respectively) for Li_(1−x)FePO₄, Li_(1−x)Fe_(0.5)Mn_(0.5)PO₄ and LiFePO₄, respectively, where 0≦x≦0.5. A plateau at 3.4 V corresponds to the Fe³⁺/Fe²⁺ redox couple and a plateau at 4.1 V corresponds to the Mn³⁺/Mn²⁺couple. With LiClO₄ in PC and DME as the electrolyte, it is only possible to charge up a cathode to 4.3 V vs. a lithium anode, so it was not possible to extract lithium from LiMnPO₄, LiCoPO₄ and LiNiPO₄ with this electrolyte. However, in the presence of iron, the Mn³⁺/Mn²⁺ couple becomes accessible. The inaccessibility is due to the stability of the Mn³⁺/Mn²⁺, Co³⁺/Co²⁺and Ni³⁺/Ni²⁺ couples in the presence of the polyanion (PO₄)³⁻. The relatively strong covalence of the PO₄ tetrahedron of the compounds of the present invention stabilizes the redox couples at the octahedral sites to give the high V_(oc)'s that are observed.

Insertion of lithium into FePO₄ was reversible over the several cycles studied. FIG. 3 shows discharge/charge curves of FePO₄/LiClO₄+PC+DME/Li coin cell at 185 mA·g⁻¹ for FePO₄ prepared by chemical extraction of Li (delithiation) from LiFePO₄. The Li_(x)FePO₄ material of the present invention represents a cathode of good capacity and contains inexpensive, environmentally benign elements. While a nearly close-packed-hexagonal oxide-ion array apparently provides a relatively small free volume for Li⁺-ion motion, which would seem to support only relatively small current densities at room temperature, increasing the current density does not lower the closed-circuit voltage V. Rather, it decreases, reversibly, the cell capacity. Capacity is easily restored by reducing the current.

As illustrated schematically in FIG. 4, lithium insertion proceeds from the surface of the particle moving inwards behind a two-phase interface. In the system shown, it is a Li_(x)FePO₄/Li_(1−x)FePO₄ interface. As the lithiation proceeds, the surface area of the interface shrinks. For a constant rate of lithium transport per unit area across the interface, a critical surface area is reached where the rate of total lithium transported across the interface is no longer able to sustain the current. At this point, cell performance becomes diffusion-limited. The higher the current, the greater is the total critical interface area and, hence, the smaller the concentration x of inserted lithium before the cell performance becomes diffusion-limited. On extraction of lithium, the parent phase at the core of the particle grows back towards the particle surface. Thus, the parent phase is retained on repeated cycling and the loss in capacity is reversible on lowering the current density delivered by the cell. Therefore, this loss of capacity does not appear to be due to a breaking of the electrical contact between particles as a result of volume changes, a process that is normally irreversible. Moreover, the problem of decrease of capacity due to the particles' breaking of electrical contact may be overcome by reducing the particle size of the cathode material to the nanometer scale.

Electrode materials with the olivine structure LiFePO₄ (triphylite) and the quasi-isomorphous delithiated material FePO₄, have the advantage of an operating voltage of 3.5V vs. Li⁺/Li⁰, i.e. in the stability window of both liquid and polymer electrolytes with a flat discharge (lithium intercalation) plateau (as seen in FIGS. 2A-2C). It may be possible to increase the diffusion kinetics and electronic conductivity by using a modified olivine structure in an electrode. The absence of non-stoichiometry or mutual miscibility for both phases (LiFePO₄ and FePO₄) in the materials with a pristine olivine structure may contribute to lower electronic conductivity than the materials having a modified olivine structure and to slower diffusion kinetics than may be achieved using materials with a modified olivine structure.

Thus, the invention additionally provides cathode materials where the pristine olivine structure of LiM(PO₄) (M=Fe or Mn or their solid solutions) is modified either on the anionic site or on the cationic site, or on both, by aliovalent or isocharge substitutions, to provide better lithium ion diffusivity and electronic conductivity. These substitutions allow for the coexistence of iron or manganese in two different oxidation states in the same phase. These substitutions may further introduce specific interactions with other elements having redox levels close to those of Fe and Mn (e.g., Fe²⁺/Ti⁴⁺⇄Fe³⁺/Ti³⁺, Mn²⁺/V⁵⁺⇄Mn³⁺/V⁴⁺, etc. . . . ) Both of these situations are favorable to electronic conductivity. Further, disorder on the anionic site provides preferential diffusion sites for Li⁺. Similarly, partial substitution of phosphorus by vanadium, or to some extent by silicon, increases the lattice parameters, thereby increasing the size of the bottlenecks which tend to slow diffusion. The formation of non-stoichiometry domains with mixed valence states and/or transition-metal mediated electron hopping, as well as partial substitution of phosphorus sites, differentiates the modified olivine compounds from the LiMPO₄/MPO₄ compounds in which the totality of Fe (Mn) is either in the +2 or +3 oxidation state.

The invention further provides new cathode materials exhibiting a rhombohedral NASICON framework. NASICON, as used herein, is an acronym for the framework host of a sodium superionic conductor Na_(1+3x)Zr₂(P_(1−x)Si_(x)O₄)₃. The compound Fe₂(SO₄)₃ has two forms, a rhombohedral NASICON structure and a related monoclinic form (Goodenough et al. 1976; Long et al. 1979). Each structure contains units of two FeO₆ octahedra bridged by three corner-sharing SO₄ tetrahedra. These units form 3D frameworks by the bridging SO₄ tetrahedra of one unit sharing corners with FeO₆ octahedra of neighboring Fe₂(SO₄)₃ elementary building blocks so that each tetrahedron shares corners with only octahedra and each octahedron with only tetrahedra. In the rhombohedral form, the building blocks are aligned parallel, while they are aligned nearly perpendicular to one another in the monoclinic phase. The collapsed monoclinic form has a smaller free volume for Li⁺-ion motion which is why the rhombohedral form is preferred. In these structures, the FeO₆ octahedra do not make direct contact, so electron transfer from an Fe²⁺ to an Fe³⁺ ion is polaronic and therefore activated.

Li_(x)Fe₂(SO₄)₃ has been reported to be a candidate material for the cathode of a Li⁺-ion rechargeable battery with a V_(oc)=3.6 V vs. a lithium anode (Manthiram and Goodenough 1989). While the sulfates would seem to provide the desired larger free volume for Li, batteries using sulfates in the cathode material tend to exhibit phase-transition problems, lowering the electronic conductivity. The reversible lithium insertion into both rhombohedral and monoclinic Fe₂(SO₄)₃ gives a flat closed-circuit voltage vs. a lithium anode of 3.6 V (Manthiram and Goodenough 1989; Okada et al. 1994; Nanjundaswamy et al. 1996). Neither parent phase has any significant solid solution with the orthorhombic lithiated phase Li₂Fe₂(SO₄)₃, which is derived from the rhombohedral form of Fe₂(SO₄)₃ by a displacive transition that leaves the framework intact. Powder X-ray diffraction verifies that lithiation occurs via a two-phase process (Nanjundaswamy et al. 1996). Increasing the current density does not change significantly the closed-circuit voltage V, but it does reduce reversibly the capacity. The reduction in capacity for a given current density is greater for the motion of the lithiated interface. The interstitial space of the framework allows fast Li⁺-ion motion, but the movement of lithium across the orthorhombic/monoclinic interface is slower than that across the orthorhombic/rhombohedral interface, which makes the reversible loss of capacity with increasing current density greater for the monoclinic than for the rhombohedral parent phase.

The cathode materials of the invention avoid the phase transition of known sulfate cathode materials by incorporating one or more phosphate ions as at least one of the constituents of the cathode material. The rhombohedral R3c (NASICON) and monoclinic P2₁/n framework structures of Li₃Fe₂(PO₄)₃ are similar to those for the sulfates described above, as illustrated in FIG. 5A and FIG. 5B.

A further embodiment of the invention is a rhombohedral NASICON cathode material having the formula A_(3−x)V₂(PO₄)₃, where A may be Li, Na or a combination thereof. Rhombohedral A_(3−x)V₂(PO₄)₃ reversibly intercalates 1.5 Li per formula unit for a discharge capacity of 100 mAh·g⁻¹ with average closed-circuit voltage being 3.8 V vs. a lithium anode (see FIG. 8). The voltage and capacity performances of the rhombohedral A_(3−x)V₂(PO₄)₃ compounds of the invention are comparable to the high-voltage cathode materials LiMn₂O₄ (4.0 V), LiCoO₂ (4.0 V) and LiNiO₂ (4.0 V). The large, open three-dimensional framework of VO₆ octahedra and PO₄ tetrahedra allows an easy diffusion of the lithium ions, making it attractive for high-power batteries. A further advantage of this material is that it includes a cheaper and less toxic transition-metal element (V) than the already developed systems using Co, Ni, or Mn.

EXAMPLES Example 1 Ordered Olivine LiMPO₄ Compounds

The ordered-olivine compound LiFePO₄ was prepared from intimate mixtures of stoichiometric proportions of Li₂CO₃ or LiOH.H₂O, Fe(CH₃CO₂)₂ and NH₄H₂PO₄.H₂O; the mixtures were calcined at 300-350° C. to eliminate NH₃, H₂O, and CO₂ and then heated in Ar at about 800° C. for 24 hours to obtain LiFePO₄. Similar solid-state reactions were used to prepare LiMnPO₄, LiFe_(1−x)Mn_(x)PO₄, LiCoPO₄ and LiNiPO₄. FePO₄ was obtained from LiFePO₄ by chemical extraction of Li from LiFePO₄. Charge/discharge curves for Li_(1−x)FePO₄ and discharge/charge cycles for Li_(x)FePO₄ gave similar results with a voltage of almost 3.5 V vs. lithium for a capacity of 0.6 Li/formula unit at a current density of 0.05 mA·cm²⁻ (See FIG. 2A and FIG. 2C). The electrolyte used had a window restricting voltages to V<4.3 V. Li extraction was not possible from LiMnPO₄, LiCoPO₄, and LiNiPO₄ with the electrolyte used because these require a voltage V>4.3 V to initiate extraction. However, Li extraction from LiFe_(1−x)Mn_(x)PO₄ was performed with 0≦x≦0.5, and the Mn³⁺/Mn²⁺ couple produced a voltage plateau at 4.0 V vs. lithium.

Arsenate may be substituted for at least some of the phosphate in LiFePO₄. The iron in LiFePO₄ may be supplemented up to 10% by other metals such as those filled with two electron shells, manganese or titanium, for example.

This iron-based material is non-toxic, inexpensive, non-hygroscopic, easy to prepare and has good electronic conductivity compared to existing high voltage cathode materials. There is a decrease in the volume of the unit cell with the extraction of lithium from LiFePO₄. At higher current densities, there is a decrease of capacity on repeated cycling associated with movement of a twophase interface, a feature characteristic of cathodes that traverse a two-phase compositional domain in a discharge cycle. The volume change across the twophase interface causes the particles to crack and lose electrical contact with the current collector. This problem can be overcome by reducing the particle size of the cathode material to the nanometer scale.

Example 2 Modified Olivine LiMPO₄ Compounds

The modified olivine compounds, LiMPO₄, were prepared from intimate mixtures of stoichiometric proportions of Fe(C₂O₄).2H₂O (Aldrich), LiH₂PO₄, Li₂C₂O₄ (Alpha Inorganics), (NH₄)₂TiO(C₂O₄)₂.H₂O and polydiethoxysiloxane (Gelest) in isopropanol. In particular, 12.95 grams of iron oxalate (Fe(C₂O₄).2H₂O), 8.31 grams of lithium dihydrogen phosphate (LiH₂PO₄), 1.52 grams of lithium oxalate (Li₂C₂O₄), 2.94 grams of ammonium titanyl oxalate monohydrate (NH₄)₂TiO(C₂O₄)₂.H₂O) and 2.68 grams of polydiethyoxysiloxane in 120 milliliters (ml) of isopropanol were ball-milled in a polyethylene container with sintered zirconia balls for two days. The resulting slurry was evaporated to dryness and treated in a vacuum furnace at 350° C., the 700° C. under argon (<6 ppm oxygen) to yield a compound with olivine structure of formula Li_(1.1)Fe_(0.8)Ti_(0.1)P_(0.8)Si_(0.2)O₄.

The compound was delithiated with a solution of bromine in acetonitrile to Li_(0.1)Fe_(0.8)Ti_(0.1)P_(0.8)Si_(0.2)O₄.

A solid-state button type cell was constructed using a 10⁶ molecular weight polyethylene oxide-lithium bis(trifluoromethanesulfonylimide) electrolyte at an oxygen/lithium ratio of 20:1; the positive electrode was made from a 45/5/50 v/v composite of respectively the olivine material, Ketjenblack and an ionic binder of composition similar to that of the electrolyte, coated on a nickel current collector. The capacity was 1.2 mAh·cm⁻². The negative electrode was a lithium disc. At 80° C., 85% of the stoichiometric discharge capacity could be obtained and cycled reversibly at 100 mAh·cm⁻².

Example 3 Rhombohedral NASICON Li_(x)M₂(PO₄)₃ Structures

The inventors compared redox energies in isostructural sulfates with phosphates to obtain the magnitude of the change due to the different inductive effects of sulfur and phosphorus. Rhombohedral Li_(1+x)Ti₂(PO₄)₃ has been shown to exhibit a flat open-circuit voltage V_(oc)=2.5 V vs. lithium, which is roughly 0.8 V below the Ti⁴⁺/Ti³⁺ level found for FeTi(SO₄)₃. The flat voltage V(x) is indicative of a two-phase process. A coexistence of rhombohedral and orthorhombic phases was found for x=0.5 (Delmas and Nadiri 1988; Wang and Hwu 1992). Li_(2+x)FeTi(PO₄)₃ of the present invention remains single phase on discharge.

All three phosphates Li₃M₂(PO₄)₃, where M═Fe, Fe/V, or V, have the monoclinic Fe₂(SO₄)₃ structure if prepared by solid-state reaction. The inventors have found that these compounds exhibit a rhombohedral structure when prepared by ion exchange in LiNO₃ at 300° C. from the sodium analog Na₃Fe₂(PO₄)₃. The discharge/charge curve of FIG. 6A for lithium insertion into rhombohedral Li_(3+x)Fe₂(PO₄)₃ exhibits an average V_(oc) of 2.8 V. This is surprisingly different from the curves for the monoclinic form (See FIG. 6B). The inventors have found that up to two lithiums per formula unit can be inserted into Li₃Fe₂(PO₄)₃, leading to Li₅Fe₂(PO₄)₃. The Li⁺-ion distribution in the interstitial space of Li_(3+x)Fe₂(PO₄)₃, where 0<x<2, appears to vary continuously with x with a high degree of disorder. FIG. 7A shows a reversible capacity loss on increasing the current density from 0.05 to 0.5 mA·cm⁻². A reversible discharge capacity of 95 mAh·g⁻¹ is still observed for rhombohedral Li_(3+x)Fe₂(PO₄)₃ at a current density of 20 mA·g⁻¹. This is much reduced compared to what is encountered with the monoclinic system (See FIG. 7B). With a current density of 23 mA·g⁻¹ (or 1 mA·cm⁻²), the initial capacity of 95 mAh·g⁻¹ was maintained in a coin cell up to the 40^(th) cycle.

Another cathode material of the present invention, Li₂FeTi(PO₄)₃, having the NASICON framework was prepared by solid-state reaction. This material has a voltage ranging from 3.0 to 2.5 V.

Rhombohedral TiNb(PO₄)₃ can be prepared by solid-state reaction at about 1200° C. Up to three Li atoms per formula unit can be inserted, which allows access to the Nb⁴⁺/Nb³⁺ couple at 1.8 V vs. lithium for x>2 in Li_(x)TiNb (PO₄)₃. Two steps are perhaps discernible in the compositional range 0<x<2; one in the range of 0<x<1 corresponds to the Ti⁴⁺/Ti³⁺ couple in the voltage range 2.5 V<V<2.7 V and the other for 1<x<2 to the Nb⁵⁺/Nb⁴⁺ couple in the range 2.2 V<V<2.5 V. It appears that these redox energies overlap. This assignment is based on the fact that the Ti⁴⁺/Ti³⁺ couple in Li_(1+x)Ti₂(PO₄)₃ gives a flat plateau at 2.5 V due to the presence of two phases, rhombohedral LiTi₂(PO₄)₃ and orthorhombic Li₃Ti₂(PO₄)₃. The presence of Nb in the structure suppresses the formation of the second phase in the range 0<x<2.

Rhombohedral LiFeNb(PO₄)₃ and Li₂FeTi(PO₄)₃ can be prepared by ion exchange with molten LiNO₃ at about 300° C. from NaFeNb(PO₄)₃ and Na₂FeTi(PO₄)₃, respectively. Two Li atoms per formula unit can be inserted reversibly into Li_(2+x)FeTi(PO₄)₃ with a little loss of capacity at 0.5 mA·cm⁻². Insertion of the first Li atom in the range 2.7 V<V<3.0 V corresponds to the Fe³⁺/Fe²⁺ redox couple and of the second Li atom in the range of 2.5 V<V<2.7 V to an overlapping Ti⁴⁺/Ti³⁺ redox couple. The insertion of lithium into Li_(1+x)FeNb(PO₄)₃ gives a V vs. x curve that further verifies the location of the relative positions of the Fe³⁺/Fe²⁺, Nb⁵⁺/Nb⁴⁺ redox energies in phosphates with NASICON-related structures. It is possible to insert three lithium atoms into the structure, and there are three distinct plateaus corresponding to Fe³⁺/Fe²⁺ at 2.8 V, Nb⁵⁺/Nb⁴⁺ at 2.2 V, and Nb⁴⁺/Nb⁵⁺ at 1.7 V vs. lithium in the discharge curve.

The rhombohedral A_(3−x)V₂(PO₄)₃ compounds of the invention can be prepared by ionic exchange from the monoclihnic sodium analog Na₃V₂(PO₄)₃. The inventors were also able to prepare the rhombohedral Li₂NaV₂(PO₄)₃ with the NASICON framework by a direct solid-state reaction (FIG. 9). The discharge/charge curves at 0.05 mA·cm⁻² (0.95 mA·g⁻¹) for the rhomobhedral Li_(x)NaV₂(PO₄)₃ are shown in FIG. 8.

The rhombohedral LiFe₂(SO₄)₂(PO₄) may be prepared by obtaining an aqueous solution comprising FeCl₃, (NH₄)₂SO₄, and LiH₂PO₄, stirring the solution and evaporating it to dryness, and heating the resulting dry material to about 500° C. Discharge/charge curves vs. lithium at 0.1 mA·cm⁻² for rhombohedral Li_(1+x)Fe₂(PO₄)(SO₄)₂, where 0<x<3, are shown in FIG. 10.

All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and structurally related may be substituted for the agents described herein to achieve similar results. All such substitutions and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.

REFERENCES

The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference:

Delmas, C., and A. Nadiri, Mater. Res. Bull., 23, 63 (1988);

Goodenough, J. B., H. Y. P. Hong and J. A. Kafalas, Mater. Res. Bull. 11, 203, (1976);

Guyomard, D. and J. M. Tarascon, J. Electrochem. Soc., 139, 937 (1992);

Long, G. J., G. Longworth, P. Battle, A. K. Cheetham, R. V. Thundathil and D. Beveridge, Inorg. Chem., 18, 624 (1979);

Manthiram, A., and J. B. Goodenough, J. Power Sources, 26, 403 (1989);

Masquelier, C., M. Tabuchi, K. Ado, R. Kanno, Y. Kobayashi, Y. Maki, 0. Nakamura and J. B. Goodenough, J. Solid State Chem., 123, 255 (1996);

Mizushima, K., P. C. Jones, P. J. Wiseman and J. B. Goodenough, Mater. Res. Bull., 15, 783 (1980);

Nanjundaswamy, K. S., et al., “Synthesis, redox potential evaluation and electrochemical characteristics of NASICON-related 3D framework compounds,” Solid State Ionics, 92 (1996) 1-10;

Nishi, Y., H. Azuma and A. Omaru, U.S. Pat. No. 4,959,281, Sep. 25, 1990;

Okada, S., K. S. Nanjundaswamy, A. Manthiram and J. B. Goodenough, Proc. 36th Power Sources Conf., Cherry Hill at New Jersey (Jun. 6-9, 1994);

Schöllhorn, R. and A. Payer, Agnew. Chem. (Int. Ed. Engl.), 24, 67 (1985);

Sinha, S. and D. W. Murphy, Solid State Ionics, 20, 81 (1986);

Thackeray, M. M. W. I. F. David, J. B. Goodenough and P. Groves, Mater. Res. Bull., 20, 1137 (1983);

Thackeray, M. M., P. J. Johnson, L. A. de Piciotto, P. G. Bruce and J. B. Goodenough, Mater. Res. Bull., 19, 179 (1984);

Thackeray, M. M., W. I. F. David, P. G. Bruce and J. B. Goodenough, Mater. Res. Bull. 18, 461 (1983); and

Wang, S., and S. J. Hwu, Chem. of Mater. 4, 589 (1992). 

1.-61. (canceled)
 62. A synthesized cathode material usable in a rechargeable electrochemical cell comprising one or more compounds, at least one compound with an olivine structure comprising the general formula LiMPO₄, where M is one first-row transition-metal selected from the group consisting of Fe, Mn, Ni and Ti.
 63. The cathode material of claim 62, where M is Fe.
 64. The cathode material of claim 62, where M is Mn.
 65. The cathode material of claim 62, wherein the cathode material comprises a second compound.
 66. A synthesized cathode material usable in a rechargeable electrochemical cell comprising one or more compounds, at least one compound with an olivine structure comprising the general formula LiMPO₄, where M is a combination of first-row transition-metals selected from the group consisting of Mn, Fe, Ti and Ni.
 67. The cathode material of claim 66, where M is a combination of first row transition-metals selected from the group consisting of Mn, Fe and Ti and where Fe_(1−x)Mn_(x) or Fe_(1−x)Ti_(x) and 0<x<1.
 68. The cathode material of claim 67, wherein the compound comprises the formula LiFe_(1−x)Mn_(x)PO₄ and 0<x<1.
 69. The cathode material of claim 66, wherein the cathode material comprises a second compound.
 70. A synthesized cathode material usable in a rechargeable electrochemical cell comprising one or more compounds, at least one compound with an olivine structure comprising the general formula LiMPO₄ where M is one or more first-row transition-metals selected from the group consisting of Fe, Mn, Ni and Ti.
 71. The cathode material of claim 70, wherein the cathode material comprises a second compound.
 72. A secondary battery comprising a cathode comprising one or more compounds, at least one compound with an olivine structure comprising the general formula LiMPO₄, where M is one first-row transition-metal selected from the group consisting of Fe, Mn, Ni and Ti.
 73. The battery of claim 72, where M is Fe.
 74. The battery of claim 72, where M is Mn.
 75. The battery of claim 72, the cathode further comprising a second compound.
 76. A secondary battery comprising a cathode comprising one or more compounds, at least one compound with an olivine structure comprising the general formula LiMPO₄, where M is a combination of first-row transition-metals selected from the group consisting of Mn, Fe, Ti, and Ni.
 77. The battery of claim 76, where M is a combination of first row transition-metals selected from the group consisting of Mn, Fe and Ti and where Fe_(1−x)Mn_(x) or Fe_(1−x)Ti_(x) and 0<x<1.
 78. The battery of claim 77, wherein the compound comprises the formula LiFe_(1−x)Mn_(x)PO₄ and 0<x<1.
 79. The battery of claim 76, where M comprises a combination of cations of the same element selected from the group consisting of Mn, Fe, Ti and Ni.
 80. The battery of claim 76, the cathode further comprising a second compound.
 81. A secondary battery comprising a cathode comprising one or more compounds, at least one compound with an olivine structure comprising the general formula LiMPO₄ where M is one or more first-row transition-metals selected from the group consisting of Fe, Mn, Ni and Ti.
 82. The battery of claim 81, the cathode further comprising a second compound. 